Behold a Gardener's Best Friend - The Earthworm
Art Frazer, Alexandria, Virginia
Reprinted with permission from Columbia Nursery's newsletter 'Azaleas'
Friend and tireless worker, at it day and night - no overtime, no union hours - a worm is an incredible creature. His underground accomplishments, and how he goes about them, and how to encourage him is worth knowing by every serious gardener.
Consuming his own weight - every 24 hours - on leaves, grass and other organic material, the earthworm fertilizes, stimulates plant growth, aerates and improves soil drainage, reduces erosion, and is a tiny but prolific factory for the production of humus.
He has no eyes or ears, but the earthworm has acute senses nonetheless. He has photoreceptor cells to enable him to detect even the faint light of dawn. Even such slight vibrations as the hop of a browsing robin ring his alarm bell. Since he has no lungs, he breathes through his skin taking oxygen from the air above ground or below ground from the air particles in good porous soil (called "structured" by the soil scientists). The deficiency of soil air is one of the reasons why there are few, if any, worms in compacted clay, and also explains why you observe worms on top of the ground after a heavy rainfall. A worm can withstand sustained droughts, losing as much as 70% of his body weight. He survives during such periods by going into a period of semi-dormancy - perhaps hibernation is a more apt description, because this is as close as this busy little worker comes to taking a vacation. Contrary to what many have believed, the earthworm does not hibernate during winter, although he does tend to carry on at deeper levels below ground. Even so, sudden surface freezes where there is no mulch, or other types of ground cover, are known to cause a considerable number of deaths from freezing.
Since my adult fascination with earthworms, I have tried to piece together their wondrous ways, through observation, reading, and research. Some of the things I have read would make a worm blush with shame, if he could read. Just one example: "chemicals" (meaning mineral fertilizers) do not kill, or discourage populations, as claimed by some "organic experts".
In my search to separate fact from myth, I eventually came upon a report by Dr. Henry Hopp, an authority on earthworms, with the U.S. Department of Agriculture. His report covered the previous work in this field augmented by his own experiments. Most of what follows about the beneficial effects of earthworms are either quotes from, or summaries of Dr. Hopp's report. But first let's start with a quotation from Dr. Hopp:
"Unfortunately, there is a great deal of misunderstanding about what earthworms do for the soil. Just as there are some individuals who, oblivious to the facts, stick to the old idea that earthworms are without real importance in farm soil, there are others who, over-enthusiastically, proclaim the earthworm a cure-all for every soil problem. Both these extreme attitudes are wrong. There are many conditions that affect the productivity of soil, and earthworms change only certain of these conditions...
"The ability of soil to produce bountiful crops depends primarily on (a) a supply of moisture in the soil, (b) an adequacy of air spaces for root development, (c) available nutrients. If any one of these three basic requirements is missing, crop growth will be poor, no matter how much of the others is present....
"Earthworm activity is not one of the three basic requirements for plant growth. Rather, it comes into the category of factors which can be used to correct deficiencies in these basic requirements. ... On some soils, it is difficult to find other practical means of doing those things which are normally done by the earthworms."
Soil consisting of course particles, such as sand absorbs (and loses) water readily. However principal soils in this area consist of much finer particles such as silt and clay. Unless they are modified by secondary agencies, the particles pack together and become almost impervious to water. Rain runs off the surface instead of entering the soil and plant life suffers from insufficient water even during normal rainfall. (At the other extreme, in low areas with poor drainage, the clay eventually becomes so water saturated that there is no soil air and plants suffocate.)
Water intake by the soil is dependent upon the presence of extraneous channels (porosity). The burrowing of worms form an interconnected network of channels which allow water to penetrate quickly through the topsoil layer. In a test plot of silt loam soil without worms it had an initial absorption of 0.2 inches of rainfall per minute. In the same plot a month after earthworms were introduced, the soil had an initial absorption rate of 0.9 inches of rainfall per minute, a fourfold improvement. Dr. Hopp points out that earthworm activity is just one of several factors that increase water absorption by soil. Mulch, compost, organic material in the soil, and the roots of plants or sod likewise increase absorption of water. However tests indicated that earthworms had much more effect on infiltration that did sod or mulch alone.
Since run-off and erosion have an inverse relationship to water absorption ability (porosity), it is not surprising that tests showed that the number of earthworms in the soil had a direct influence on erosion.
Dr. Hopp says that compact soils - again soils largely clay and/or silt - sustain plant life poorly because there are not enough air spaces in the soil. Not only is water absorption slow, as we have seen, but aeration is deficient. Plants grow best in well aerated soil. Not only is needed oxygen supplied to the roots, but new roots grow primarily in air spaces between the soil particles.
"Earthworms are one of the most effective agents for loosening and aerating the soil. Their burrows make large passageways for the roots to grow in. They perforate the topsoil especially, and gradually penetrate the subsoil, opening it for root growth and depositing organic material in it. But even more important is the granulation of the soil which they bring about. This is done by their production of casts from the soil and organic debris that they eat. As the soil becomes granulated with casts, it gets looser and looser...During damp seasons of the year, cast production is especially prolific."
Based on the observation that earthworms produce casts equivalent to their own weight each day, it appears that 700 lbs. of casts per acre per day is generated in soil during damp weather with an average earthworm population.
Plowed soil is temporarily loosened, but with eventual rains, if it is poor (unstructured) soil, it packs together again. The earthworm has it all over the plow due to the type of granulation that his castings produce. Soil scientists recognize that the "water-stability" of soil is one of its most important physical characteristics. Water-stability is the result of cementing soil particles together by sticky materials not readily dissolved by water. They are produced by life in the soil, such as earthworms and certain micro-organisms.
Quoting Dr. Hopp directly again: "Earthworms affect the nutrient supplying ability of the soil by taking organic debris from the surface (or in the ground) and incorporating it into the topsoil. They digest the debris and excrete in their casts what nutrients they do not need for their own nutrition...so distributed through the root zone, the casts constitute a source of nutrients for vegetation."
Dr. Hopp explains that the richness of the casts depends on the kinds of organic debris and mineral soil available for earthworm food. However a comparative analysis of earthworm casts and uneaten soil made by Lunt and Jacobson at the Connecticut Experimental Station is little short of amazing. They reported that the casts contained: 5 times the nitrates, 7 times the available phosphorus, 3 times the exchangeable magnesium, 11 times the potash, and 1½ times the lime (calcium) than occurred in uneaten soil from the top 6 inches of a field. These amazing statistics have been widely quoted. It is unfortunate that they have frequently been quoted out of context or distorted to support otherwise unsubstantial claims.
It is obvious, as Dr. Hopp points out, that the startling increases in available nutrients came from the organic debris which the earthworms ate. To clinch his point Dr. Hopp ran experiments in mineral soil alone (no organic material) to show by comparative analysis that there was no difference in available mineral soil eaten by earthworms and that not eaten.
THE EARTHWORM'S LITTLE HELPERS:
The earthworm isn't the only working denizen of the down under. Several kinds of microscopic life also break down organic material into humus. These microorganisms include bacteria, fungi. and tiny insects. We could get along without worms, but not without microorganisms. However they do not operate as effectively without worms.
The microorganisms needed for the manufacture of humus from organic material are, generally speaking, found in ordinary soil in fabulous quantities. What they do in the soil is the same chore they accomplish in a compost pile. In fact they are, of course, essential in the current experiment at the U. S. D. A. Research Service at Beltsville, Md. to compost sewage sludge into a practicable agricultural material.
Having learned all the wonderful things the busy little earthworm does for our soil, how can we make him happy - and encourage him to multiply? First and foremost, we must provide a good home. The earthworm population is known to vary from zero to a couple million per acre depending upon the character of soil, adequate moisture, and organic content. Differences in earthworm populations in three nearby samples are interesting:
Beltsville, Md. - 350,000 per acre
Frederick, Md. - 2,200,000 per acre
Morgantown, W. Va. - 1,200,000 per acre
Contrary to some advertising, it is futile to buy earthworms to populate your garden. If you don't have worms in your garden, something is wrong. It may be the kind of soil, the lack of organic material, or even perhaps - though not likely - the presence of toxic chemicals. If you take the trouble to correct the basic problem, the earthworms will come to work for you. But it's up to you to provide the fringe benefits.
We, the residents of suburbia, and the metropolitan areas in general, are most often confronted with a property of sticky, gooey, unproductive clay. Why it is unproductive has just been explained. What's to be done?
For one thing we can excavate the whole mess and buy several loads of topsoil. But the topsoil most often turns out not to be that dark rich brown earth of grandpa's day, heavily larded with humus. Instead it most often is worn out, or tired old soil, scraped off some future shopping center, basically not much better than what we had to begin with. Or we can buy several loads of sand to laboriously mix with our clay to make a more porous structure - but this doesn't take care of the needed humus content.
Our answer here at the nursery - and around our residence, is based on the premise that clay is a satisfactory mineral medium, if we can develop it into "structured soil" - remember Dr. Hopp's criteria. This we do by copious infusions of organic "debris", leaves, sawdust, peat moss - supplemented with modest amounts of nitrogen fertilizer to replace the nitrogen all decomposing organic material utilizes. We accomplish this by first plowing the soil with a rototiller, then spreading 6 to 8 inches of sawdust, and finally rototilling the sawdust until it is thoroughly mixed with the loose soil. Contrary to what is often cautioned, we do not distinguish between well-rotted and fresh sawdust - a lesson we learned several years ago from Mr. James Wells. (see James S. Wells, "Plant Propagation Practices", Macmillan Co.) As a matter of fact we now prefer fresh sawdust because it decomposes more slowly. As mentioned we sprinkle nitrogen fertilizer (ammonium sulfate) on the sawdust before tilling it in, at the rate of roughly 4 lbs. per 100 sq. feet.
What does this accomplish? Many things: (1) it breaks up the clay and prevents compaction; (2) it provides much greater aeration (porosity); (3) it provides better drainage, and at the same time better retention of moisture ("capillary water" that surrounds soil particle); (4) it provides soil rich in organic material, which azaleas and many other plants love; and (5) IT ATTRACTS AND ENCOURAGES EARTHWORMS, which do all the things we described earlier. We regularly use mulch to do all the things that mulches do, plus provide organic food for our little humus makers. We have followed this routine for 12 years, and whenever we replant a section of the nursery we usually plow in more leaves, or sawdust depending upon what is at hand. We recognize that organic content in average good soil is about 5%. Our most recent soil analysis showed that our organic content ranges from 7.8 to almost 14%.
The most striking personal illustration of this approach occurred two years ago when we enlarged the nursery to include a piece of uncultivated land. The soil was hard compacted clay, larded with sizeable river rock. We practically wore out one rototiller - and almost wore out a husky wrestler-football player - on that small plot of land. One thing was for sure - not an earthworm in sight. By the following spring earthworms abounded - you couldn't dig a plant without turning up 2 to 6 worms.
So in our experience, soil conditioning, as we practice it, and earthworms go hand in hand. If there is a simple way to summarize all that we have just been saying - and provide a good rule of thumb for your guidance, it is this reminder. Good soil is only about 45 percent minerals ("dirt" particles) and about 5 percent, or more, organic material; the other 50 percent - if the soil is to be productive - must be about evenly proportioned between moisture (25%) and air (25%). Obviously such proportions vary somewhat from place to place, or even from time to time (so far as the moisture-air proportions are concerned). But if these proportions get too badly out of kilter, the health and vigor of your plants will be affected.